1. Field of the Invention
The present invention relates to power tools and more particularly, relates to power tools, such as impact wrenches and impact screwdrivers.
2. Description of the Related Art
Japanese Laid-open Patent Publication No. 6-304879 describes an impact wrench that can be used firmly tighten fasteners, such as a bolt or nut This known impact wrench has an output shaft (drive shaft) and a hammer that strikes the output shaft. Generally speaking, a socket is attached to a distal end of the output shaft A fastener may be disposed within the socket. Then, the output shaft is forcibly rotated in order to tighten the fastener within or to a workpiece. The hammer is allowed to slip and freely rotate with respect to the output shaft when a predetermined amount of torque is exerted. Thus, when a load for rotating the output shaft is light (i.e., before the fastener becomes seated against the workpiece), the hammer continuously rotates the output shaft in order to continuously tighten the fastener. However, after the head of the fastener has contacted the workpiece (i.e., after the fastener has become seated against the workpiece), the hammer will begin to slip and rotate freely. Therefore, the hammer will impact the output shaft after rotating by predetermined angle. By repetition of the slipping and impacting action, the output shaft will rotate a small amount each time the hammer impacts the output shaft and the fastener can be tightened to an appropriate torque.
This known impact wrench further includes an impact detecting sensor that detects whether the hammer is distant from the output shaft (i.e., whether the hammer slips with respect to the output shaft), and a rotational angle detecting sensor that measures the rotational angle of the output shaft The impact detecting sensor outputs an OFF signal when the hammer is in an engaged state with the output shaft, and outputs an ON signal when the hammer is distant from the output shaft. The rotational angle detecting sensor outputs a signal that corresponds to the rotational angle of the output shaft. A controller of the impact wrench detects changes in the rotational angle of the output shaft in the period between the impact detecting sensor outputting one ON signal and outputting a subsequent ON signal, and determines from the changes in the rotational angle of the output shaft whether the tightening torque of the fastener has reached a predetermined value (i.e., whether the fastener has become seated against the workpiece). When the tightening torque reaches, the predetermined value, the controller begins to detect changes in the rotational angle of the output shaft from that point in time again. When the detected changes in the rotational angle reach a preset value, the motor is stopped. Consequently, after the fastener has become seated against the workpiece, the fastener is further tightened until the changes in the rotational angle reach the preset value. As a result the fastener can reliably be tightened by means of this impact wrench.